Hydrocarbons sometimes exist in a formation but cannot flow readily into the well because the formation has very low permeability. Acidizing wells is a conventional process for increasing or restoring the permeability of subterranean formations so as to facilitate the flow of oil and gas from the formation into the well. This process involves treating the formation with an acid to dissolve fines and carbonate scale that are plugging or clogging the pores, thereby opening the pores and other flow channels and increasing the permeability of the formation. Continued pumping forces the acid into the formation, where it etches channels or wormholes. These channels provide ways for the formation hydrocarbons to enter the well bore.
Conventional acidizing fluids, such as hydrochloric acid or a mixture of hydrofluoric and hydrochloric acids, have high acid strength and quick reaction with fines and scale nearest the well bore, and have a tendency to corrode tubing, casing and downhole equipment, such as gravel pack screens and downhole pumps, especially at elevated temperatures. In addition, above 200° F. (92° C.), HCl is not recommended in some cases because of its destructive effect on the rock matrix. Due to the type of metallurgy, long acid contact times and high acid sensitivity of the formations, removal of the scale with hydrochloric acid and hydrochloric acid mixtures has been largely unsuccessful. However, there are other acid fluid systems to dissolve carbonate and/or scale and remove the source of the fines through acidizing the surrounding formation while not damaging the downhole equipment, particularly for high temperature wells. These acid systems include, but are not necessarily limited to mixtures of tricarboxylic acids, aminocarboxylic acids, dicarboxylic acids including, but not necessarily limited to, oxalic acid (ethanedioic acid), malonic acid (propanedioic acid), succinic acid (butanedioic acid), glutaric acid (pentanedioic acid), adipic acid (hexanedioic acid), pimelic acid (heptanedioic acid), and mixtures thereof. Further details about these acids may be had with reference to U.S. Pat. No. 6,805,198 assigned to Baker Hughes Incorporated, and it is incorporated herein by reference.
Other acids used to treat subterranean formations include, but are not necessarily limited to sulfuric acid, phosphoric acid, hydrofluoric acid, hydrobromic acid, fluoroboric acid, formic acid, acetic acid, glycolic acid, and mixtures of these and the forementioned acids.
It is also known to use emulsified fluids containing such acids to treat subterranean formations. Emulsified acids give reduced or retarded acid reaction rates because of the oil-external outer phase. Some of these acids mentioned above have reduced acid reaction rates with carbonate reservoirs that in turn allows deeper and narrower wormholes to form that may include secondary microfractures. The longer wormholes and/or secondary microfractures are needed in order to improve hydrocarbon production rates. However, a difficulty with some emulsified acids is that they are not thermally stable, that is the viscosity changes markedly over time at elevated temperature.
It would be desirable if a composition and method could be devised to overcome some of the problems in the conventional acidizing methods and fluids, particularly with respect to thermal stability.